Start by boiling a large pot of water for the pasta. Once the water is boiling, add the pasta and simmer for 12 minutes until it’s fully cooked.
While the pasta cooks, heat the oil in a large frying pan over medium-high heat. Season the salmon fillets with salt and pepper, then place them in the pan, skin-side up.
Cook the salmon for 3 minutes until golden, then flip it over and push it to one side of the pan. In the open space, add the onions and cook over medium heat for 3-4 minutes, stirring occasionally, until they soften.
Add the minced garlic and cook for an additional 30 seconds. Then stir in the passata, tomato puree, dried thyme, and sugar. Mix everything together and let it simmer for 2-3 minutes.
The pasta should be almost done by now. For the last 30 seconds of cooking, add the spiralized courgette (zucchini) to the pan with the pasta. Once cooked, drain the pasta in a colander, but make sure to reserve a cup of the cooking water.
Flip the salmon in the frying pan and, using tongs, carefully remove the skin and discard it. Flake the salmon into chunky pieces with the back of a fork, then stir the cream into the sauce.
Add the cooked spaghetti and spiralized courgette to the pan, along with a splash of the reserved pasta cooking water (about 1/4 cup). Use tongs to toss the spaghetti in the sauce, making sure the salmon is evenly distributed throughout.
If you prefer a thinner sauce, add more pasta cooking water to loosen it up. Divide the pasta into bowls and top with a generous sprinkle of grated Parmesan, black pepper, and fresh basil leaves.